Why do atomic nuclei decay?

Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus). ... The nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. read more

This, by itself, does not cause an atom to come apart, because if it could do that, the atom would not come together to begin with. But it can make an atom very unstable. What the atom needs to decay is two things, this instability, and some unpredictable event. The actual event can any of a variety of things, but two can provide examples. read more

A = atomic mass (number of protons + neutrons) Z = atomic number (number of protons) X = chemical symbol (as shown on the Periodic Table) Alpha (α) decay. When an alpha particle is emitted from a nucleus the nucleus loses two protons and two neutrons. This means the atomic mass number decreases by 4 and the atomic number decreases by 2. read more